The invention relates to an apparatus for filtering and separating flow media by way of membranes, particularly in accordance with the methods of ultrafiltration, reverse osmosis and nano-filtration, comprising an essentially pressure-tight housing in which the membranes are arranged and which includes an inlet for admitting the flow medium to be separated into the housing and outlets for discharging the permeate and the retentate from the housing.
Such an apparatus is known for example from EP-8-0289740. In this apparatus, the medium to be separated is conducted over a stack of individual membrane elements past which the medium flows via flow passages between the membrane elements, which are supported in spaced relationship by spacer elements on which the membranes are supported. The membrane elements are in the form of so-called membrane pillows and the flow medium flows past the stacked membrane pillows in a meander-like flow pattern from the inlet of the apparatus to the outlet.
The permeate itself is collected in the interior of the membrane pillows, is conducted to a central permeate discharge opening of the permeate pillow and from there is discharged by way of a permeate collection pipe extending through the apparatus.
Because of the meander-like flow path for the flow medium around the individual membrane elements from the inlet of the apparatus to its outlet, through which the flow medium leaves the apparatus in a concentrated state that is as a retentate, there is a substantial pressure loss from the inlet to the outlet, which pressure loss is larger, the larger the number of membranes stacked on top of one another in the apparatus. Such apparatus can generally only be operated reasonably if sufficient energy is available to sufficiently pressurize the flow medium to be separated upon entering the apparatus. The operation of such apparatus, which as regards their separating function have been quite effective during practical use, consumes therefore a large amount of energy. Although it is possible theoretically to improve the output of such apparatus by increasing the number of membrane elements per apparatus, this also increases the energy requirements of the apparatus because of the principle explained above.
It has been tried to reduce the energy consumption of such apparatus while maintaining the same membrane surface area by not conducting the flow medium through the apparatus in a meander-like flow pattern from the inlet to the outlet but conduct the flow medium in a parallel flow pattern past membrane elements arranged one after the other, that is, by providing a flow path which is essentially linear with the membrane elements oriented in the flow direction. A typical example of such an apparatus is described in EP-B-0 707 884.
When there was a need for increased performance also in this case, an additional set of membrane elements was installed in the apparatus essentially linearly with the flow direction. With this increase of the effective membrane surface in the apparatus a modular-like extension of the apparatus could be provided in a basically elegant manner.
This apparatus is different from the first-mentioned apparatus in that it actually had substantially lower energy requirements for the same separating performance than the first mentioned apparatus.
However, both types of apparatus are limited with respect to the total available membrane surface area that can be accommodated by the apparatus because of their respective designs which permits accommodation of only a limited amount of membrane elements.
For certain applications of such apparatus, for example, in commercial and military ships and boats, there is, on one hand, a need for low energy requirements of such apparatus and, on the other hand, for high efficiency, that is, a high separation performance in combination with low space requirements.
It is therefore the object of the present invention to provide an apparatus in which the advantages of the two types of apparatus are combined. It should accommodate a large membrane area and have a relatively low hydraulic resistance for the flow medium passing through the apparatus and it should be simple in design so that its construction costs are low.
It should furthermore be easy to replace present apparatus of the type referred to above by the new apparatus without the need of increased space and it should have an improved separation performance.